1. Field of the Invention
The present invention relates to a semiconductor device and a method of fabricating the same. More particularly, the invention relates to a semiconductor device having a reduced floating inductance and a method of fabricating the same.
2. Description of the Background Art
FIG. 15 shows a conventional composite substrate 50 with thick copper foil for a power module in top plan view, in cross section and in bottom plan view.
The composite substrate 50 includes a ceramic insulating substrate 51, a circuit pattern 52 deposited on one surface of the ceramic insulating substrate 51, and a back pattern 53 deposited on the other surface thereof.
In FIG. 16, there is shown, in perspective, a conventional semiconductor device 60 including the composite substrate 50. The semiconductor device 60 comprises a copper base plate 1, the composite substrate 50, a power semiconductor element 12, aluminum wires 13, main electrode terminals 62, a control electrode terminal 63, and a case 61.
FIG. 17 shows a main current path 64 of the semiconductor device 60 in practical use. It will be apparent from the comparison between FIGS. 16 and 17 that the main current path 64 from one of the main electrode terminals 62 leads to the semiconductor chips 19 and then runs through the wires 13 and part of the circuit pattern 52 to exit at another main electrode terminal 62.
Such a semiconductor device is used as a power switching device, repeating ON/OFF operation. As MOS power devices have been recently put to practical use, the switching frequency is very high, commonly in the range of 20 to 30 KHz. The switching frequency is expected to increase.
As the switching frequency increases, the rate of current change per time di/dt rises in ON-to-OFF and OFF-to-ON operation. This causes a surge voltage V=L.multidot.di/dt in ON-to-OFF operation due to a floating inductance L present in the main current path 64. An increase in the floating inductance L results in an increase in surge voltage. When the increasing surge voltage exceeds the rating of the power semiconductor elements 12, the power semiconductor elements 12 break down.